The heat treatment mentioned above may advantageously be applied locally so as to dissolve—partially or completely—the oxide or oxynitride layer in defined regions of the SeOI structure, corresponding to a desired pattern, while preserving the initial oxide or oxynitride layer in other regions. This is then referred to as “local dissolution” of the oxide or oxynitride layer.
The expression “oxynitride” is understood to mean a compound having the general formula (Se)OxNy where Se is the symbol of the semiconductor considered (for example silicon) and where x and y are the non-zero oxygen and nitrogen contents, respectively. The oxide corresponds to the case where y=0.
By virtue of such a heat treatment, it is possible to obtain, as illustrated in FIG. 1, an SeOI structure having a variable-thickness oxide or oxynitride layer (in the case of a partial dissolution) or even, as illustrated in FIG. 2, a hybrid structure, i.e., comprising both “SeOI” zones, in which the oxide or oxynitride layer has been preserved, and zones in which this layer has been totally dissolved.
In the case of an oxynitride layer, nitrogen also diffuses through the thin layer of semiconductor, so that after the dissolution treatment, the oxide or oxynitride is transformed into the semiconductor considered.
The SeOI structure of FIG. 1 comprises a support substrate 1, an oxide or oxynitride layer 2 the thickness of which has been locally reduced by the dissolution treatment, and a thin semiconductor layer 3.
The hybrid structure of FIG. 2 comprises a support substrate 1 and a thin semiconductor layer 3, between which the oxide or oxynitride layer 2 has been preserved in certain regions (allowing “SeOI” zones to be formed), and completely dissolved in others (allowing bulk semiconductor zones referenced B to be formed).
Such a structure may be employed to manufacture electronic components (for example “memory” components and logic components) that require different substrates on one and the same wafer.
In other words, it enables the co-integration of circuits that respectively operate on SeOI substrate and on bulk substrate within one and the same chip.
The advantage of local dissolution is therefore to provide a manufacturer of integrated circuits with a wafer comprising “bulk” and “SeOI” zones on which they will be able to fabricate, while preserving their tried and tested technologies, both components requiring a bulk zone and components requiring an SeOI zone.
Specifically, the precision of the local dissolution technique makes it possible to define bulk zones and SeOI zones at the component scale.
Local dissolution is generally implemented by forming a mask on the surface of the thin semiconductor layer and then by applying the heat treatment that promotes the diffusion of the oxygen from the oxide or oxynitride layer towards the surface of the semiconductor layer.
Since the mask is in general made of a material that forms a total or partial barrier to oxygen diffusion, the oxygen can diffuse easily only through the exposed zones of the thin semiconductor layer, i.e. those zones not covered by the mask. In the case where the mask allows partial diffusion of the oxygen, it nevertheless ensures a much lower dissolution rate than that of the exposed (mask-free) zones in which the dissolution is easier because of the absence of mask.
Document WO 2008/114099 describes such a process, in which the mask is obtained by oxidation and completely blocks diffusion. The use of such a mask may have certain drawbacks, however, including the appearance, in the thin semiconductor layer, of trenches at the mask edge. These trenches may have several origins: wetting of the mask by the semiconductor, reaction between the mask and the semiconductor, etc. In every case, it is the high surface mobility of the semiconductor atoms that is responsible for the creation of these trenches. This surface mobility depends on the temperature and the reducing or weakly oxidizing atmosphere of the treatment. These topographical defects, the depth of which may reach the thickness of the semiconductor layer, are detrimental to the fabrication of components on the thin semiconductor layer.
In particular, the mask-edge defects amplify the surface topography variation. This variation makes subsequent circuit fabrication steps difficult to carry out and may lead to dewetting of the semiconductor, i.e., to a loss of cohesion in the thin semiconductor layer, which dissociates so as to form droplets on the surface of the oxide or oxynitride layer.
To remove or minimize these topographical defects, a chemical-mechanical polishing, with the aim of planarizing the surface (so as to prevent level differences related to the sagging of the semiconductor layer), is difficult to implement because it would remove too great a thickness of the semiconductor layer, as the initial thickness of this layer is already chosen to be small so as to facilitate oxygen diffusion. Furthermore, the polishing tends to reduce the thickness uniformity of the semiconductor layer.
A process for locally dissolving the oxide or oxynitride layer which does not have the aforementioned drawbacks is therefore sought. One aim of the invention is thus to provide a local-dissolution process after which the surface topography of the thin semiconductor layer is improved.